1. Field of Invention
This invention relates to combustion control systems and more particularly to a new combustion control system which uses chemiluminescence of the flame to control the flow of fuel and air to the primary combustion reaction zone.
2. Related Art
The efficiency of many heat transfer processes is directly related to the efficiency of a combustion reaction. Although many factors such as fuel atomization, reaction zone temperature and content of volatile material in the fuel influence combustion efficiency, one of the most important factors is fuel air ratio. For a given quantity of fuel having a fixed content, a theoretical amount of oxygen must be available for complete combustion of the fuel. Combustion under these conditions is termed stoichiometric combustion. If insufficient oxygen is provided, not all of the fuel will be combusted resulting in decreased combustion efficiency. If excess oxygen is provided, some of the heat generated by combustion is used to heat the excess oxygen again resulting in decreased efficiency. In addition to the thermodynamic efficiency of the process, another important consideration is the reduction in pollutants emitted from the combustion process. By accomplishing complete combustion of fuels, pollutant emissions such as carbon monoxide and soot are minimized. The combined combustion aspects of thermodynamic efficiency and complete combustion to reduce pollution emissions can be termed flame quality.
In modern utility fossil fired boilers, complicated analog or digital control systems are used to regulate fuel air ratio to obtain optimal flame quality. Generally, these systems measure the flow of fuel and combustion air to the furnace and adjust one or the other to obtain the correct fuel air ratio. The measurement of fuel flow has a number of difficult problems. If the fuel is liquid, such as No. 6 fuel oil, a venturi or turbine flow meter is the normal method of measurement. To obtain accurate measurement with these instruments, it is usually necessary to mount them in a pipeline having a sufficient length of straight pipe upstream and downstream of the instrument to avoid inaccuracies caused by disparate crosssectional velocity profile within the pipe. In addition, since the venturi or turbine flow meter is basically a volume measuring device, density corrections such as temperature compensation must be added to accurately convert the volume measurement to a mass measurement. If the fuel is solid, such as coal, the fuel flow measurement becomes even more difficult. In coal fired utility boilers, fuel flow measurement is generally accomplished by weigh scale feeders used to transport coal to the pulverizers (mills). These feeders cannot discriminate between coal and scrap material such as dirt or clay which is always present to some degree in the coal fed to the pulverizers. In addition, variations in moisture content of the coal significantly impact the measurement. Even if complicated compensation systems are added to the control system to correct for the variables described above, the content of major combustion reactants, carbon and hydrogen, in a given quantity of fuel (liquid or solid) can vary significantly. Presently, there is no satisfactory method of real-time measurement of these parameters. Accurate measurement of combustion air flow has difficulties similar to those associated with fuel flow. Combustion air flow in a utility boiler is generally measured in one or two large supply ducts. The measurement device is influenced by turns and bends in the duct upstream and downstream of the device. To eliminate this influence, designers attempt to have sufficient straight runs of ducting before and after the device. The length of the necessary straight runs is related to the crosssectional area of the duct and is rarely available for the designer in a typical, compact boiler plant layout. Air flow measurement accuracy is also degraded by leaks in seals and ductwork downstream of the measurement point. Also, the need to convert volume measurement to mass measurement described above is applicable to the air flow measurement.
To account for some of the problems outlined above, combustion control designers have utilized post combustion flue gas analysis measurement to correct or trim the fuel air ratio. Usually these systems measure the oxygen, carbon dioxide or carbon monoxide content of the flue gas. If the flue gas contains excessive oxygen, too much combustion air is being delivered to the combustion zone resulting in decreased efficiency. If the flue gas contains excess carbon monoxide, insufficient combustion air is being delivered thereby preventing complete combustion to carbon dioxide and again limiting the efficiency of the combustion process. If sufficient information is available on the carbon content of the fuel being burned, carbon dioxide measurement of the flue gas can also be used to trim fuel air ratio to the optimum point. Flue gas analysis devices are complicated and require substantial maintenance to achieve an acceptable reliability for a utility boiler combustion control system. The flue gas analysis devices are generally located in ducts downstream of the furnace and therefore analyze the combined gases from all of the burners in the boiler. Thus, if one burner of a multiple burner installation is operating with an inefficient fuel air ratio, the device may not detect the inefficient operation and certainly could not determine which burner was causing the inefficiency. In addition, most utility boilers utilize induced draft fans to withdraw combustion gases from the furnace. The induced draft fans cause the interior of the furnace to operate at a slightly negative pressure compared to surrounding atmospheric pressure. Thus any leaks in the furnace casing cause excess air to enter the flue gas path downstream of the combustion zone leading to artificially high oxygen content readings.
One solution to the above described problems is to measure the combustion efficiency or flame quality right at the flame front. J. M. Beer et al in their report (Beer, J. M., Jacques, M. T., and Teare, J. D., "Individual Burner Air-Fuel Ratio Control: Optical Adaptive Feedback Control System," M.I.T. Energy Laboratory Report No. MIT-EL-82-001, 1982) discuss the use of spectrometric measurements of the emissions of ultraviolet and infrared radiation from the flame front as an indicator of flame quality and combustion efficiency. These investigations found that using a single detector and monochromator directed at a single region of the flame and turned to the spectral frequency associated with radiation emitted from the OH radical provided repeatable and accurate information on combustion efficiency within a narrow range of burner load. Significant difficulties were encountered when this approach was used over a wide burner load range. The monochromator was positioned and focused to collect emission data from a single small region near the burner end. The detector used with the monochromator produces an analog output proportional to total emission in the frequency range of interest within the monochromator's field of view. Consequently, as burner load changed, the flame geometry and position, as influenced by aerodynamic flow variations, significantly impacted the measurement of OH emissions. As burner load increased, the higher flow of fuel and air shifts the location of OH radical production within the flame envelop and therefore makes the measurement system extremely sensitive to burner load. In addition, since the monochromator in effect measured the average emission from the focal plane, it could not recognize variations of emissions from different parts of the focal plane.
In a similar approach using chemiluminescence to monitor flame quality, E. Gutmark et al (Gutmark, E., Parr, T. P., Hanson-Parr, D. M., and Schadow, K. C., "Use of Chemiluminescence and Neural Networks in Active Combustion Control," Proc. of 23rd Symposium (Int.) on Combustion (Pittsburgh: The Combustion Institute), 1101, 1990) showed that measurement of CH radicals were an effective measure of flame quality. To improve accuracy of the measurement, the system included a soot measurement instrument and used six variations of the CH radical and soot readings including, average CH, average soot, root mean square CH, peak CH, peak soot and CH/soot relative phase. A neural network was developed to emulate the operation of the laboratory type burner used to test the system. The neural network emulator used inputs from the fuel delivery equipment comprising fuel flow fluctuation frequency and amplitude which were the controlled variables in the experiments. Although the neural network emulator was successful at modeling the CH output from the flame, it did not account for the previously described problems associated with varying burner load and the resulting changes in flame geometry and position.
The problems associated with flame geometry and positioning were recognized and addressed in U.S. Pat. No. 4,555,800 issued to Nishikawa et al on Nov. 26, 1985. This patent discloses an imaging system used to categorize flame patterns by their geometry. The system captures an image of the flame and compares its geometry to a set of image standards which have been developed and stored in a computer beforehand. The image standards have known carbon monoxide (CO) and nitrogen oxide (NOx) content for diagnosing the state of the flame in regard to these parameters. The patent does not disclose how the image standards are developed, but it is apparent that the accuracy of the system is dependent on the empirically derived relationship between the flame geometry and CO and NOx content. The flame images record the entire visible spectrum emitted by the flame, including soot, hot particles or ash, and visible gas-phase emitting species. The system, therefore, is not sensitive to gas chemistry alone and may easily be confounded by changes in soot or particulate loading. In addition, since flame shape is highly dependent on the geometry of the burner equipment, windbox and furnace, image standards would have to be developed for each combustor installation to achieve accurate results.
Consequently, there is still a need for an accurate and effective measurement of combustion efficiency and flame quality which can be made right at the burner combustion flame front and are specific to critical reaction species.